Artificial lift is comprised of methods used to transport produced fluids to the surface when reservoir pressure alone is insufficient. Gas lift is a common method that is particularly suited to high-volume offshore wells. A high pressure (up to several thousand psi) gas is injected into the tubing by a casing annulus and travels to a gas lift valve. The operating valve provides a pathway for a designed volume of gas to enter the production tubing. The gas reduces the density of the fluid column, decreasing backpressure on the producing formation. The available reservoir pressure can then force more fluid to the surface. Gas lift valves are effectively pressure regulators and are typically installed during well completion. Multiple gas lift valves may be required to unload completion fluid from the annulus so injected gas can reach the operating valve.
Gas lift is known to be effective and gas lift wells have generally been proven to be low maintenance. One issue, however, is that gas lift wells remain operational even when they are not optimized. That is to say that a non-optimized gas lift well will typically still flow production fluids, albeit at a reduced production rate, even if it is receiving too much, or too little, gas lift gas and/or is lifting from multiple valves or a valve above the operating point. Field diagnostics and modeling have estimated that less than 25% of gas lift wells are optimized, resulting in lost production and inefficient allocation of lift gas.
Gas lift performance is typically gauged through periodic well testing. A test separator is commonly used to measure the volume of liquid and gas produced by a well. A typical well test can take four to twelve or more hours. If a change is made to a well, that is the gas lift rate is increased or decreased, a production choke is adjusted, etc., the well may need several hours to stabilize at a new operating condition before it can be re-tested. A typical method of optimizing a gas lift well is to select a gas lift rate and test the well, and then repeat the process with additional gas lift rates until a gas lift performance relationship is obtained. The most effective/economic gas lift rate can then be selected. Unfortunately, offshore facilities have a limited amount of space for equipment, so a given production platform or vessel may only have one to two test separators for deployment. If each well must be tested at least monthly for regulatory purposes, the test separator may not be available for gas lift optimization.
Other, more technical options for gas lift optimization are not always feasible. For instance, gas lift wells can be modeled with inflow and outflow software and producing pressures and temperatures can be compared to the models. However, this strategy requires accurate models and pressure and temperature transducers located in the flow path. Additionally, the software and sensors must be maintained and periodically recalibrated to ensure accuracy, as well performance changes over time. Individual well, multiphase flow meters could be installed, but these tools are relatively expensive and new to the industry.
As such, there exists a need to address the aforementioned problems and issues. Therefore, what is needed are simpler solutions for gas lift optimization and systems for their implementation.